Electrowinning Aluminum in the Hall-Heroult Cell
Since the patenting of the Hall-Heroult cell ("HHC") in 1886 for electrowinning liquid aluminum (Al) at about 960.degree. C., the basic features have remained the same, although obviously significant optimization of the process variables has occurred. (1) Even today, liquid Al is deposited into a carbon (cathode) hearth, having sidewalls protected by frozen crust, by electrochemically reducing alumina (Al.sub.2 O.sub.3) dissolved in a fused fluoride electrolyte.
The principal component of the fused electrolyte is cryolite (Na.sub.3 AlF.sub.6), although the NaF/AlF.sub.3 bath ratio has been optimized and other bath additions (e.g., LiF, CaF.sub.2, MgF.sub.2) have been made. The electrolyte serves as the solvent for alumina derived from bauxite ore, typically purified by the Bayer digestion process. Most important as backdrop for the process improvement disclosed herein, the modern version of the HHC runs the anodic oxidation reaction at an expensive prebaked and refined carbon anode, resulting in the oxidation and consumption of the carbon to release CO.sub.2 product gas. The present invention will also apply to the replacement of the older, but still currently used process, involving a Soderberg carbon anode.
As is well understood by the industry worldwide, there are many problems associated with the use of the consumable carbon anode. First, the stoichiometric consumption of the carbon anode, according to the reaction EQU 1/2Al.sub.2 O.sub.3 +3/4C=Al+3/4CO.sub.2 ( 1)
represents a significant cost for the carbon, amounting to about 14.4% of the cost of producing primary Al. (2) However, the formation of the CO.sub.2 gaseous product from carbon oxidation offers the advantage that the thermodynamic (open-circuit) voltage for Eq. (1) is held down to 1.20 volts; but then the anodic oxidation reaction has a significant overvoltage of about 0.5 volts, while another 0.35 volts are required to pass the high current through the anode. Furthermore, the uneven oxidation of carbon results in a rounding of the originally flat anode geometry which necessitates a significant anode-to-cathode spacing of about 5.0 cm (to avoid shorting) and thereby a significant IR drop (about 1.45 volts) through the electrolyte, requiring periodic anode adjustment. The evolution of CO.sub.2 bubbles at the carbon anode also introduces an additional polarization of about 0.30 volts, while some back reaction between the CO.sub.2 product and the reduced aluminum lowers the current efficiency for the particular materials and process variables used. Since the electrical cost of the electrolysis process is directly proportional to both the applied cell voltage and the current efficiency, the elimination, or minimization, of certain of these contributions to the cell voltage could lead to a significant reduction in the cost of producing primary aluminum, as will be demonstrated later.
As an additional important factor opposing the continued use of the carbon anode, the release of the greenhouse gas CO.sub.2 by the process is meeting increasing environmental objection. While the stoichiometric requirement for anode carbon according to Eq. (1) is 0.33 #C/#Al, in fact, direct oxidation and other losses result in the consumption of about 0.45 #C/#Al, amounting to the release of 1.65 #CO.sub.2 /#Al.(2-5) Further, from an environmental standpoint, in addition to CO.sub.2, the fabrication and oxidation of carbon anodes also evolve objectionable HF, CO, perfluocarbon volatiles, and other volatile organic compounds (VOC). (5) The equipment and associated maintenance and labor to reduce these emissions inherent to the use of the carbon anode represent a significant cost and problem for the primary aluminum producers.
Through Faraday's Law, the rate of Al production is established (for 100% current efficiency) by the cell current. But a significant cost of electrical energy is required for electrowinning Al. The current US composite baseline energy use is estimated to be 15.2 kWh/kg Al. About 22.8% of the total cost is proportional to the impressed cell voltage which constitutes the summation of the thermodynamic (open-circuit) voltage, the anodic and cathodic overpotentials, bubble effects, the IR drop in the fused salt electrolyte, plus voltage drops in the electrodes and collector bars external to the cell, etc. (4)
The modern HHC operates today at about 4.4 volts with a current efficiency of about 95%. The heat balance for the cell is maintained by providing sufficient insulation so that the I.sup.2 R heat generated in the electrolyte keeps the cell at the operating temperature of about 960.degree. C.
The maximum permissible anodic current density, which limits the rate of Al production, is set by the occurrence of the "Anode Effect." When the local concentration of alumina dissolved in the electrolyte becomes too low, CO.sub.2 evolution at the anode is interrupted, a passivating/insulating film of very environmentally objectionable fluorocarbon (CF.sub.4 and C.sub.2 F.sub.6) species cover the anode, and the cell must receive immediate attention (i.e., in order to force alumina replenishment to the electrolyte) to resume production. Of course, the Anode Effect cannot be blamed solely on the nature of the anodic oxidation reaction on carbon, but also on the difficulty to dissolve sufficient alumina rapidly enough to support the anode reaction.